| The substantial role of land-climate interaction in the earth system has been widely and long-term concerned. However, until recently less attention has been paid to the relationship between soil moisture and climate changes over East Asian. With the monthly mean soil moisture in-stu dataset and ERA40 soil moisture reanalysis data, the interannual variations, trend changes and the spatial characteristic of soil moisture over East China have been studied. Based on the temporal and spatial characteristic of soil moisture over East China, the impact of soil moisture on the East Asian summer monsoon has been evaluated in detail too. And ensembles of seasonal simulations (March-September) have been performed in order to investigate the sensitivity of the Asian summer monsoon and the rainfall over eastern China to regional soil moisture anomalies. The main conclusions are as follows:1. In the past 45 years (from 1958 to 2002), the spring soil over eastern China exhibit evidently drying trend. The soil over south China and northeastern China has a consistent drying variation from top to deep-layer. This kind of trend becomes marked after 1980 in South China. In northeastern China, the shallow-layer soil drying trend weakens while the deep-layer soil becomes more distinctly during 1970~1996. In mid-latitude of China, the shallow-layer soil moisture exhibit significant periods of 3~5 year and the deep-layers soil dry markedly after 1988. In a word, the drying trend of soil is more significant in South and northern China than mid-latitude of China, and it is more evident in deep-layer than shallow-layer.2. The 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA40) soil moisture dataset generally reproduced the temporal and spatial characteristic of the in-stu dataset. Since soil moisture observations are limited both in time and space, and are generally not representative on the regional scale and long-time scale, ERA40 soil moisture dataset is a good alternative for research work.3. The spring soil moisture over South China has a significant negative correlation with the summer rainfall over the lower reach of Yangtze River valley during 1958~2002. It suggested that the persistently positive anomaly of rainfall in lower reach of Yangtze Rive vally after 1980 probably has relationship with the long-time drying variation of soil over South China. The summer rainfall in North China decline sharply during 1970s last century. The event may correlate the spring soil moisture marked drying trend over northeastern China. The correlation between summer rainfall over North China and spring soil moisture over northeastern China is significant positive during 1958~2002. The spring soil moisture decreases evidently over the two areas during the past 20 years, which weakens their impact on the summer monsoon over East China and strengthens the influence of soil moisture in the mid-latitude area (from North China to the lower reach of Yangtze River valley) to the summer rainfall.4. East Asian summer monsoon rainfall in the East China is very sensitive to spring soil moisture in the area from North China to the lower reach of Yangtze River valley (NCYR). The summer precipitation in the northeastern China and Yangtze River have a positive anomaly with increasing spring soil moisture in NCYR, while summer rainfall in South China and North China have a negative anomaly. The spring soil moisture in NCYR influences the water and energy equilibrium on land surface and thereby the atmosphere through land/atmosphere interactions. The soil moisture positive anomaly decreases the land-sea temperature contrast through cooling land surface, which weakens East Asian monsoon. The west pacific subtropical high (WPSH) is located to the south and shifts westward, which hampers the northward motion of East Asian monsoon, causes the rainfall belt southward, and thereby produce more precipitation over the lower reach of Yangtze River valley. Conversely, the rainfall over South China decreases because of the stronger WPSH.5. The precious spring soil moisture anomaly also has an important impact on the trough and ridge system in mid-high latitude. The elevated soil moisture induces more evaporation, which provides more latent heat flux and less sensible heat flux. More latent heat flux and less sensible heat flux means a more potential instability energy of low level atmosphere, which induces a westerly current long wave trough between 100°E and 120°E and lead to a positive anomaly rainfall in northeastern China.6. The growth of vegetation in NCYR is sensitive to the previous soil moisture anomaly. The increasing soil moisture in spring is propitious to the growth of vegetation and leads to a higher vegetation cover in summer, which modulates the water, heat and energy exchange between land and atmosphere and tranquilizes climate. 7. A series of experiments with NCEP/GSM (global spectral model) coupled with SSiB surface model have been conducted to understand the influence of soil moisture on East Asian summer monsoon and the monsoon rainfall in East of China. The experiments are designed to illustrate the dominant features of monsoon and the monsoon rainfall anomaly response to soil moisture anomaly in NCYR. The results indicate that the impact of soil moisture anomalies on the monsoon appears nonlinear with respect to wet and dry events. The monsoon is weakened in the wet experiments but change less noticeably in the dry experiments. The wetter land surface conditions (enhanced soil moisture) in NCYR cold the land surface and low-lever atmosphere, which decrease the land-sea temperature contrast and change the summer monsoon circulation. The equatorial westerly monsoon flow weakens in the lower troposphere and strengthens in the upper troposphere over a broad region including tropical Asia, the Indian Ocean and part of the western Pacific Ocean. Over mid-high latitude, the upper westerly is much stronger and too southward. The summer monsoon becomes significantly weaker. The WPSH extends westward and hampers the northward motion of East Asian monsoon, and thereby produce more precipitation over the low reach of Yangtze River valley. Conversely, the rainfall over northern and South China decreases.8. Soil moisture affects the East Asian summer monsoon rainfall through two competing processes: 1) the"soil-precipitation"feedback, local enhancement of precipitation with a large evaporation during to the higher soil moisture and 2) a thermodynamical effect whereby surface evaporation cools the land surface, decreases the land-sea temperature contrast, and thereby weakens the monsoon flow and change the monsoon precipitation. The second process determines the feedback between soil moisture and monsoon rainfall in the wet experiments. However, the monsoon changes much less in dry experiments and the first process control the feedback. The dry soil produce less latent heat flux and more sensible heat flux (big Bowen ratios), which imply the buildup of a comparatively deep boundary layer and lead to a low values of low-lever moist entropy. The process act in concert to decrease the potential for convective activity and lead to less rainfall over most of area in East of China.
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